Mast cells adhere to fibronectin following aggregation of high affinity receptors for IgE (Fc epsilon IR) on the membrane surface. This process is mediated by an alpha-5 containing integrin. Fc epsilon RI-induced adhesion is transient (compared to stem cell factor-induced adhesion), and occurs through 'inside-out signaling' which results in a conformational change in the surface integrins but does not alter their number. Mast cells spontaneously adhere to vitronectin. This event is followed by the phosphorylation of multiple intracellular proteins, a process which can be detected as early as 5 minutes after adhesion. Among the phosphorylated proteins is focal adhesion kinase (FAK). FAK is also phosphorylated after aggregation of Fc epsilon RI, or after the addition of c-kit ligand, also termed stem cell factor (SCF). IL-3 dependent mast cells undergo apoptosis (programmed cell death) following removal of IL-3. This is prevented by the addition of SCF. Addition of TGF-beta prevents this SCF-mediated rescue, thus demonstrating one way in which the microenvironment can control mast cell numbers. Mouse bone marrow-derived mast cells respond to both IL-3 and c-kit ligand and are inhibited by m-CSF, GM-CSF, and gamma-IFN. Mouse peripheral blood mononuclear cell-derived mast cells respond only to c- kit ligand. Human peripheral blood-derived human mast cells are derived from a CD34+ Fc epsilon RI- cell population. CD34 positivity is rapidly lost in culture, while the cell population becomes Fc epsilon RI+. The culture of human mast cells is enhanced if human serum is used instead of fetal calf serum. Fibrinoblasts in vitro selectively support the survival of mast cells, while internalizing and degrading basophils.